Light-emitting diodes (LED's) are being used to replace standard lighting to reduce energy consumption. LED's are commonly used as backlighting for liquid crystal displays (LCD's) such as in large displays and television sets, and for smaller displays and projectors, and other applications. A LED-based projector may have Red, Green, and Blue LED's that are switched on and off rapidly to generate light for pixels that may be generated in the optical path. Moving Micro-Electro-Mechanical System (MEMS) micro-mirrors, LCDs, or Liquid Crystal on Silicon (LCoS) may reflect the LED-generated light under control of pixel display data.
Display refresh rates, quality, and resolutions may require that the LED's switch on and off at a high rate, such as 1 MHz. The amount or intensity of light produced depends on the current through the LED's. Relatively high currents may be required for driving the LED's. Special LED driver circuits may be used to drive the LEDs.
FIG. 1A shows a LED driver. LED driver circuit 100 is powered by power supply voltage Vcc. The LED voltage LED_V produced by LED driver circuit 100 is usually above the forward-biased diode drop according to the desired current when LED 10 is turned on by switch 11, and kept constant when LED 10 is turned off by switch 11. A control signal LED_SW applied to switch 11 at the cathode of LED indicates when to turn LED 10 on and off.
LED 10 may have several LED's in parallel, and may have several (N) LED's in series, requiring a higher LED voltage such as N times the diode drop. The LED current LED_I produced by LED driver circuit 100 can be very high when high intensity light is produced.
FIG. 1B highlights an image sticking problem with inadequate LED drivers. It can be quite a challenge to design LED driver circuit 100 to quickly turn the LED current LED_I on and off. When the LED_SW control input is switched on, the LED current LED_I cannot instantly switch on. Instead, LED_I rises over a period of time. The sudden switching on of LED_I can cause the LED voltage LED_V to initially dip, preventing LED 10 from turning on quickly. The intensity of the visible light produced by LED 10 has a delay in rising.
A similar problem can occur when LED_SW is turned off, causing the visible light to remain on for a period of time after the ideal switch point. This extra delay 102 can cause visible artifacts, such as image sticking, an incorrect light and color combination. For example, the red LED can linger on for extra delay 102, causing some red to be added to the next pixel being displayed by the imager panel. When the next pixel is blue, that next pixel may appear purple instead of blue. The slow response due to image sticking may be visible or perceptible to the human eye.
Other problems from extra delay 102 may include loss of brightness and loss of detail. The switching of the large LED current may also cause dips and spikes in the power-supply voltage Vcc and the driver output voltage LED_V. Ringing, oscillation, or ripple may occur on the node due to inductances and parasitic resistances and capacitances. In some applications, the driver output voltage is also shared with other circuits. This ringing may affect circuits in unusual ways, further degrading image performance. Some driver circuits may be effective for low-going transitions but produce excessive delays and ripple for high-going transitions, or vice-versa.
What is desired is a LED driver circuit that reduces current-switching problems such as image sticking. An LED driver that has digital control is also desired. An LED driver that uses digital control of currents to reduce power-supply and ground noise, ringing, and oscillation is also desirable.